1. Field of the Invention
This invention relates to a method and apparatus for controlling a nuclear reactor under dropped control rod conditions, and more particularly for shutting down the reactor only if the dropped rod condition causes the axial power distribution to exceed prescribed limits.
2. Prior Art
One means for controlling the reactivity of a commercial nuclear reactor such as a pressurized water reactor (PWR) is through insertion of control rods into the reactor core. The control rods absorb neutrons to control the number of fission reactions. Since the control rods are inserted into and withdrawn from the upright, generally cylindrical core along a vertical path, they have a direct effect on the axial distribution of the fission reactions, and hence the power generated, in the core. Skewing of the power generated in the core in the axial direction due to the effect of the control rods is commonly measured in terms of a quantity such as axial offset or axial shape index which reference skewing of power toward the top and bottom of the core respectively. Operational constraints placed on the axial distribution of power in the core are translated into limits on axial offset or axial shape index which, if exceeded, lead to tripping or shutting down of the reactor through full insertion of all the control rods.
PWRs have both full length control rods which extend all the way through the core and part length rods which cover only portions of the core. The full length rods may be inserted to any depth in the core while the part length rods are either fully inserted or fully retracted. In those reactors in which the part length rods are manipulated during power changes, the position of these rods makes a substantial difference in the critical limits on axial offset or axial shape index.
Both the full length and part length control rods are inserted and retracted in groups consisting of rods located symmetrically about the vertically oriented axis of the generally cylindrical core such that normal movement of the control rods does not cause an imbalance in the radial distribution of power. However, the rods are incrementally stepped into and out of the core by electromechanical jacks which can, on occasion, malfunction resulting in the dropping of a rod into the core, thereby distorting the axial distribution of power. The nature of the operation of a PWR is such that the reactor attempts to make up for the local loss of reactivity caused by the dropped rod by increasing the power elsewhere in the core so that the demand placed upon the reactor is still met. This in turn, can lead to local limits being exceeded in these other parts of the core.
One type of protection system provided on PWRs monitors the neutron flux and trips the reactor when a negative rate of change of flux in excess of a preselected value is detected. Such a negative rate of change in neutron flux can for example, indicate a dropped rod since the inserted rod reduces the local reactivity and it takes time for the power to increase elsewhere in the core and thus, return the power to the demanded level. The control rod drives in PWRs are such that dropping of control rods may occur during an operating cycle. The current protection system for these plants will usually respond to a dropped rod by tripping the reactor. The combined effect of several trips could result in a significant capacity factor loss.
It is not necessary, however, to trip the reactor if local power peaks can be maintained below design limits. Accordingly, commonly owned U.S. Pat. No. 4,399,095 proposes that while the reactor should be tripped upon the occurence of a very large negative flux change, for a more moderate drop in power the reactor can continue to operate as long as the power does not exceed the reduced power level initially resulting from the dropped rod by a preselected amount. It also calls for limits on rod withdrawal and a rollback in turbine power to maintain reactor power below the new limit.
It is a primary object of the present invention to provide a method and apparatus for controlling a nuclear reactor which permits the reactor to operate at full power with a dropped rod as long as local peak power in the core does not exceed design limits.